From the prior art, numerous active substance release systems based on resorbable polymers are known which are injected or implanted. These systems are always preferred when the active substance has to be released over a fairly long period and oral administration is impossible or is not sufficiently reliable or effective. A resorbable implant which contains active substance is also preferred to oral administration when the active substance has to be released within a specific area of the body, as in the case, for example, with powerful cytostatics used on tumours.
Implantable active substance carriers should satisfy the following criteria:
The active substance should be released at a constant rate over a fairly long period of time and the polymer carrier should be broken down within a reasonable time so that there is no need for the implant to be surgically removed after the active substance has been released. Important parameters in connection with this are the swellability (water uptake), the reduction in molecular mass and the reduction in the mass of the polymer.
Processes for preparing poly-D,L-lactide, e.g. by polymerisation of lactide in the presence of suitable polymerisation catalysts (higher molecular masses) or by the polycondensation of lactic acid (lower molecular masses) are part of the prior art.
The term "poly-lactic acid" in this case refers to polymers synthesised from lactic acid units and having a low level of polymerisation. Poly-lactic acids of this kind are usually prepared by condensation of lactic acids but are also obtained in the ring-opening polymerisation of lactides under suitable conditions.